In offshore operations in which a drill pipe is in a locked-to-bottom mode from a floating vessel, a heave-compensator system is used aboard the vessel to maintain a constant tension on the drill pipe. However, if a situation arises in which the compensator locks, a heave by the vessel will generate a very high tension on the drill pipe. Another challenge of a floating vessel relates to situations in which the vessel may uncontrolledly start to drift away from an optimum positioning over a well which is connected via a drill-pipe string from said vessel, which, in turn, will lead to the vessel generating a very high tension on the drill pipe. Such drifting may be due to breakage in one or more anchors or faults in digital positioning systems for the floating vessel which is equipped with such digital positioning systems. The tension on the drill pipe will presumably either break the drill pipe or cause serious damage to the Christmas tree or wellhead or to the vessel. Irrespective of which part is broken or damaged in such a situation, fatal situations may arise.
To avoid such a fatal situation it is known to provide a weak point in the drill pipe. Such a weak point, which is also known by persons skilled in the art as a “weak link”, may be provided by reducing the cross-sectional area of the drill pipe at a predetermined level in the given drill string, represented by one of the drill-pipe sections. Alternatively, or additionally, a so-called “shear sub” of limited capacity may be arranged inside an underwater BOP (blowout preventer).
Both the weak link and the shear sub have several drawbacks. The tensile capacity of the drill string cannot be changed while the drill string is in operation, or immediately before an installation operation is started. Further, the weak link represents limitations to the overall operation, especially with respect to torsion, bending and tensile capacities. The latter is especially a drawback if a scenario with a stuck drill pipe arises, in which the drill pipe will have to be freed by means of pulling. A further drawback is that it may be challenging to retrieve the lower part of a drill string after a drill string has been torn apart at said weak link. This is a time-consuming and expensive operation. Yet another drawback is that the tensile capacity of a weak link must be tested with respect to mechanical properties of the pipe material.
The above-mentioned drawbacks have led to several inventions which provide solutions that may be alternatives to said weak link. Most of these inventions relate to a disconnecting system for high-pressure riser systems, which could, in principle, have been used for operations in which a drill-pipe string is used. A challenge of said disconnecting systems is high complexity to ensure safety, and further to prevent emissions of hydrocarbons into the external environment as high-pressure riser systems are used in operations in which a floating vessel is connected to an active well. The high complexity may be a drawback in itself because of technical unreliability.
Offshore operations in which a drill pipe is in a locked-to-bottom mode from a floating vessel are often performed in connection with pressureless wells so that there is a lower risk with respect to safety and external environment than what is typical of operations with riser systems. It may thus be appropriate to have disconnecting systems that have a lowest possible technical complexity for reliability to be ensured, while at the same time, safety is ensured and damage to critical equipment is avoided if one or more of said challenges of a floating vessel should arise. Further, it turns out in several cases that it may be appropriate to place such a disconnecting system as close to the seabed as possible to avoid leaving behind a relatively long drill string on top of subsea equipment such as a Christmas tree or a BOP. A long drill string projecting up from the top of said subsea equipment after a release will entail drawbacks and challenges. Such drawbacks and challenges will be known to a person skilled in the art and thus are not explained any further here.
In this connection it should be mentioned that, in several operations, said drill string, also referred to as a drill pipe in what follows, will be run on the inside of a drill-pipe-and-riser system (marine riser) which involves physical limitations on a disconnecting system which is to be placed as close to the subsea equipment as possible. Further, in most cases, there is a need to be able to extend control lines that are necessary for the operation of said subsea equipment from the vessel along the drill pipe on the inside of said drill-pipe-and-riser system.
U.S. Pat. No. 5,382,056 discloses a coupling with a weak connection for a riser string. The weak connection includes an arming device. The arming device allows the coupling with the weak connection to split at a minimum axial force only when an operator puts the arming device in an armed position.
SU 560965 discloses a quick-release for drill pipes, in which a connection between two pipes is provided by means of male and female portions including tapering helical threads. The connection is released when a tensile force is above a preset value.
U.S. Pat. No. 3,842,914, US 2009/0301711 and U.S. Pat. No. 3,148,894 disclose connections for drill pipes which are deactivated by an object being dropped from the surface of the well.
US 2010/0282474 A1 discloses a coupling for use on a pipe in connection with platforms at sea. The coupling includes a first portion and a second portion which are displaceably engaged with each other and which are releasably attached to each other by means of a hydraulically activated locking element.
WO 2011/074984 A1 discloses a release module for attaching a pipe string in a heave-compensated, load-bearing unit in a derrick on an offshore platform.
WO 2013/071983 A1 discloses a disconnecting system for high-pressure riser systems including a release device which is activated if a tensile force exceeds a preset threshold force, and a release device which is activated if a compressive force exceeds a preset threshold force.
US 2011/0127041 discloses a disconnecting system for high-pressure riser systems including a release device and a pressure-application unit adapted to apply a coupling force to at least partially eliminate a separation force applied as a result of well pressure. The well-pressure separation force acts to separate upper and lower parts of the release device.
US 2014/050522 discloses a connection for transmitting rotational forces from a rotating drilling machine to a drill string in a so-called horizontal drilling process. A so-called “saver sub” is used as a connection to transmit the rotational forces when a drill string is run into or out of a borehole.
US 2012/205118 discloses a tensioning apparatus for applying a substantially constant tension to a workover riser. The tensioning apparatus includes a first part which is adapted to be coupled to a workover riser, a second part which is adapted to be coupled to a marine riser and a tensioning device which provides relative movement between the first part and the second part to tension the workover riser.
NO 334034, belonging to the present applicant, discloses a coupling apparatus and a method of connecting two drill-pipe sections in a string of drill-pipe sections joined together by tool joints. The coupling apparatus includes a first coupling portion provided with a first pipe which has a first end portion and a second end portion, the first end portion being provided with a tool joint for connection to a drill-pipe section; a second coupling portion provided with a second pipe which has a first end portion and a second end portion, the first end portion being provided with a tool joint for connection to a drill-pipe section. The first and second coupling portions are arranged for displaceable engagement until the second end portion of the first pipe has been joined to the second end portion of the second pipe to provide a continuous bore through the apparatus, the apparatus further including an engagement device for releasably attaching the first coupling portion to the second coupling portion and transmitting, through the apparatus, an axial load on the string of drill-pipe sections. The engagement device is provided with a mechanically adjustable control means for controlling an axial-load-carrying capacity of the apparatus, the control means including a movable element arranged for movement in an axial direction of the apparatus between a first position and a second position. The apparatus functions satisfactorily, but involves a relatively complex structure.